All the VJ softwares actually works like that,
you play a piece of video in loop, having the cursor automatically or manually sliding from a certain T-Start to T-End
i'm working on a user interface for processing JS which should allow to dynamically manually change the sliding range of several parameters according their ranges which may also vary during the performance

here it goes,
this comes from Resolume software (one of the most famous on windows platform)
per each channel as you can see you got 'limit cursors' (the two rectangles extreme left and right) which allows you to set the slider range, than the main cursor which moves continuously based on the settings (play, pause, loop, reverse, random,...)

in fact i'm working on a widget which should implement the whole element which is such a basic fundamental brick in the whole solution